Branched-chain amino acid biosynthesis has been most thoroughly characterized in Bacillus subtilis; hence, most of the information in this chapter is from studies of B. subtilis. The novel mechanisms for biosynthesis in other gram-positive genera are also described. In addition, the organization of the genes responsible for isoleucine, valine, and leucine biosynthesis and proposed regulatory mechanisms for expression of those genes is elucidated and compared with those for the corresponding genes in the gram-negative bacterium Escherichia coli. The chapter illustrates the major pathways for the synthesis of the branched-chain amino acids in vegetatively growing B. subtilis cells. In Bacillus spp., some of the intermediates in branched-chain amino acid biosynthesis, specifically the branched-chain keto acids, are the immediate precursors of synthesis of the branched-chain fatty acids that are an integral part of the cell membrane. The chapter also talks about the regulation of ilv and leu gene expression. A brief overview of what occurs in E. coli is presented first, and then what occurs in B. subtilis is described. The genetic organization and regulation of the ilv and leu genes are significantly different in grampositive and gram-negative bacteria. As in E. coli, B. subtilis cells increase expression of the ilv and leu genes when there is a need for the branched-chain amino acids. Much more investigation is needed before the complete story of branched-chain amino acid biosynthesis in the gram-positive bacteria is known.

Nucleotide sequence of the upstream regulatory region of the B. subtilis ilvB gene. Underlined sequences represent the -35 and -10 segments of the promoter, +1 indicates the transcription start site, and dots signify the Shine-Dalgarno site before the ATG start of the ilvB open reading frame. Arrowheads indicate regions of inverted repeats occurring in transcripts that originate at the promoter. These sequences can form potential stem-and-loop structures A:B and C:D (shown in Fig. 5).

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10.1128/9781555818388/fig22-4.gif

Figure 4

Nucleotide sequence of the upstream regulatory region of the B. subtilis ilvB gene. Underlined sequences represent the -35 and -10 segments of the promoter, +1 indicates the transcription start site, and dots signify the Shine-Dalgarno site before the ATG start of the ilvB open reading frame. Arrowheads indicate regions of inverted repeats occurring in transcripts that originate at the promoter. These sequences can form potential stem-and-loop structures A:B and C:D (shown in Fig. 5).

Predicted secondary structure of the terminator that occurs in the untranslated leader transcript upstream of the B. subtilis ilv-leu gene cluster. Structure shown is the hairpin A:B, the terminator, which was identified in Fig. 4. C and D represent inverted repeats (arrows) that are capable of base pairing and might interfere with the terminator structure. The base change that occurs in segment A of one azlA mutant, azlA102, is indicated.

10.1128/9781555818388/fig22-5_thmb.gif

10.1128/9781555818388/fig22-5.gif

Figure 5

Predicted secondary structure of the terminator that occurs in the untranslated leader transcript upstream of the B. subtilis ilv-leu gene cluster. Structure shown is the hairpin A:B, the terminator, which was identified in Fig. 4. C and D represent inverted repeats (arrows) that are capable of base pairing and might interfere with the terminator structure. The base change that occurs in segment A of one azlA mutant, azlA102, is indicated.

Nucleotide sequence of the DNA upstream of the B. subtilis ilvA gene. Dots indicate the Shine-Dalgarno site preceding the ATG start codon. Arrowheads indicate a region of DNA that can potentially form a stem-and-loop structure. Codons in the amino-terminal end of the coding region that are specific for leucine are underlined.

10.1128/9781555818388/fig22-6_thmb.gif

10.1128/9781555818388/fig22-6.gif

Figure 6

Nucleotide sequence of the DNA upstream of the B. subtilis ilvA gene. Dots indicate the Shine-Dalgarno site preceding the ATG start codon. Arrowheads indicate a region of DNA that can potentially form a stem-and-loop structure. Codons in the amino-terminal end of the coding region that are specific for leucine are underlined.

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